Two different facilities are for example described in the references DE 10 2005 062 528 A1 and DE 10 313 127 B4. In the last-mentioned reference a method is described in which the lower side of the wafer is contacted with the etching solution, wherein a meniscus is formed. During the whole process time the lower side of the wafer is continuously in contact with the etching solution. In the first-mentioned reference a method is described in which the lower side of the wafer is not continuously in contact with the etching solution. The etching solution is applied onto the lower side of the wafer by a special profile pattern of the rotating transport rolls, wherein the wafer at the same time is horizontally moved on. At the lower side of the wafer a liquid film is formed.
Through the etching reaction inter alia nitrogen oxides (“NOx” gases) are produced which are partially dissolved in the etching solution, but which also partially reach the headspace of the process modules. Nitrogen oxides in connection with vapors of hydrofluoric acid have an etching effect on silicon, in particular on highly doped silicon, such as the emitter, and through the HF portion also on phosphorus silicate glass or borosilicate glass. For avoiding an undesirable (since nonuniform) etching of the emitter side, the reaction vapors are withdrawn by suction from the process compartment and/or additives are added to the process medium for avoiding the formation of gas.
In DE 10 2009 050 845 A1 a method is described in which onto the upper side of the wafer before the one-sided etching process a defined amount of water is applied onto the wafer. The water spreads out across the hydrophilic wafer surface (with phosphorus silicate or borosilicate glass) and thus forms a protection layer against the etching vapors.
In DE 10 2008 037 404 A1 a one-sided etching process is described in which the composition of the gas phase above the process medium is influenced in such a manner that the surface during the process of edge isolation is made hydrophobic and/or after a previous treatment in diluted hydrofluoric acid stays hydrophobic. Only acidic media are described.
It is also possible to use the reaction vapors for an intentional back-etching of the emitter. Such a process is described in WO 2011/035748 A1. In order that the emitter side is etched as homogenously as possible, the reaction vapors are accordingly withdrawn by suction or an inert gas is fed to the gas phase over the process medium in addition or a reactive gas such as HF or ozone is fed in addition. For back-etching of the emitter optionally an additional etching step, preferably with HF and ozone, as solution or in the gas phase, is mentioned for a better adjustability of the process of back-etching. A uniform controlled back-etching of the emitter is desirable, because by the removal of highly doped topmost emitter layers the surface recombination of the produced charge carriers is reduced and thus the efficiency factor of the solar cells is improved. Normally, the edge isolation and the back-etching of the emitter are conducted in two separated process modules with different etching solutions. The etching media which are proposed are acidic.
The NOx/HF vapors have to be aftertreated separately in suitable facilities. Fluoride is precipitated as CaF2 by bubbling through lime milk, the NOx gases are reacted to nitrogen. The etching solution of HNO3/HF is toxic. Therefore, the disposal of spent etching solutions of HNO3/HF is costly.
In all mentioned and industrially used methods an etching solution on the basis of hydrofluoric acid and nitric acid is used.
An alkaline process for the removal of the undesired doped region on the back side is described in US 2004/0259335 A1. Here the front side and the edges are protected by a protection layer/mask of silicon nitride. The etching rate of silicon nitride in an alkaline solution is negligible low. But a disadvantage of alkaline solutions is that the tendency of creeping is still higher than in the case of acidic solutions.
A reliable practicable alkaline method for one-sided etching of a substrate, in particular for edge isolation, without the need of a mask for the protection of the emitter side is not known.